Cold rolled steel sheets used for the production of shaped articles through deep drawing, such as cold rolled steel sheets for deep drawing and the like, are required to be low in the yield strength (YS) and high in the elongation (El), that is, to be excellent in the ductility, and further to have a high Lankford value (r-value) as important mechanical properties. Moreover, shaped articles produced through deep drawing are often used in the outer surface of mechanical products, such as automotive exterior plate and the like, and therefore it is an important property for the cold rolled steel sheet to have an excellent surface property.
Cold rolled steel sheets for deep drawing have hitherto been produced from a low-carbon aluminum killed steel through a box annealing. However, recently a continuous annealing method is widely used for the production of the cold rolled steel sheets for deep drawing from the low-carbon aluminum killed steel in view of the improvement of productivity and the energy saving. However, the use of commonly used low-carbon aluminum killed steel as a starting material can not give satisfactorily excellent mechanical properties to the resulting cold rolled steel sheets for deep drawing. Accordingly, it was proposed to use ultra-low carbon steel having a C content of as low as 0.020% or less as a starting material for the production of cold rolled steel sheet for deep drawing. However, it was difficult to secure, in the conventional method, satisfactorily high r-value and ductility enough to bear the deep drawing even in the use of such ultra-low carbon steel.
Under these circumferences, there have been proposed various methods, wherein carbide- and nitride-forming elements of Nb, Ti, Zr and the like are added to ultra-low carbon steel. Among the prior arts disclosing these methods, Japanese Patent Application Publication No. 18,066/69 and Japanese Patent Laid-open Specification No. 137,021/78 disclose cold rolled steel sheets containing Ti and having deep drawability, and methods of producing the steel sheets.
However, in these methods, it is necessary to carry out a hot rolling at a high finishing temperature, and a high temperature heating of slab and a high temperature hot rolling must be carried out. However, the high temperature heating of slab has such drawbacks that cost for heating energy is high, yield is low due to the oxidation of slab surface, quality of the resulting cold rolled steel sheet is poor due to the increase of internally oxidized product, and troubles occur during the cold rolling. While, the high temperature hot rolling is apt to cause breakage and other troubles of roll and to deteriorate the quality of the surface of the resulting cold rolled steel sheet.
Further, Japanese Patent Laid-open Specification No. 13,123/82 discloses a method of producing a cold rolled steel sheet for deep drawing from a steel containing C: 0.002-0.05% and Ti: 0.070-0.210% through a low-temperature hot rolling. However, in this method, a large amount of Ti is used, and therefore the resulting cold rolled steel sheet is very expensive, and further the cold rolled steel sheet is poor in the surface property and in the treatable property of surface due to the increase of Ti series inclusions.
The object of the present invention is to provide a method of producing cold rolled steel sheets adapted for deep drawing and having excellent ductility and surface property from a Ti-containing steel through a low-temperature hot rolling.